CVE-2023-25139: GNU Glibc Buffer Overflow Vulnerability

CVE-2023-25139 Overview

CVE-2023-25139 is a critical buffer overflow vulnerability affecting the sprintf function in the GNU C Library (glibc) version 2.37. The vulnerability allows an out-of-bounds write to occur even when the destination buffer is allocated with what would normally be considered the correct size. This flaw manifests when attempting to write a padded, thousands-separated string representation of a number, causing the function to write beyond the allocated buffer bounds.

Critical Impact

This vulnerability enables attackers to potentially execute arbitrary code or cause denial of service through memory corruption when applications use sprintf with locale-specific number formatting. The network-accessible attack vector with no authentication requirements makes this particularly dangerous for networked applications.

Affected Products

• GNU glibc 2.37
• Systems and applications using glibc 2.37 for number formatting with thousands separators
• NetApp products (see NetApp Security Advisory NTAP-20230302-0010)

Discovery Timeline

• February 3, 2023 - CVE-2023-25139 published to NVD
• March 26, 2025 - Last updated in NVD database

Technical Details for CVE-2023-25139

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write) and represents a particularly insidious type of buffer overflow. Unlike typical buffer overflow vulnerabilities where developers fail to allocate sufficient memory, this flaw triggers even when the buffer is sized correctly according to standard calculations.

The root issue lies in how sprintf handles padded, thousands-separated numeric output. When a developer calculates the buffer size needed for a formatted number (including thousands separators and padding), the calculation may be mathematically correct, yet glibc 2.37's implementation writes additional bytes beyond this calculated size.

For example, formatting the number 1,234,567 with padding to 13 characters results in a two-byte overflow beyond the theoretically correct buffer allocation. This makes the vulnerability particularly dangerous because it can affect security-conscious code that has properly calculated buffer requirements.

Root Cause

The vulnerability stems from an implementation error in glibc 2.37's sprintf function when handling locale-specific formatting with thousands separators and padding. The internal buffer size calculations within the formatting routines do not correctly account for the interaction between grouping characters (thousands separators) and field width padding, leading to writes that exceed the calculated buffer boundaries.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can potentially trigger this vulnerability by:

1. Providing malicious input to applications that format user-controlled numeric data using sprintf with locale-specific thousands separators
2. Targeting applications that accept numeric input and display formatted output
3. Exploiting web applications, APIs, or network services that perform number formatting on untrusted data

The out-of-bounds write can overwrite adjacent memory, potentially corrupting control flow data structures, function pointers, or other sensitive memory regions. This could lead to arbitrary code execution, application crashes, or privilege escalation depending on the memory layout of the target application.

Technical details and discussion of this vulnerability can be found in the Sourceware Bug Report #30068 and the Openwall OSS Security Notice.

Detection Methods for CVE-2023-25139

Indicators of Compromise

• Unexpected application crashes or segmentation faults in processes using glibc number formatting functions
• Memory corruption errors in applications processing numeric data with thousands separators
• Abnormal behavior in applications after processing formatted numeric output
• Core dumps showing corruption near sprintf or related formatting function calls

Detection Strategies

• Implement runtime memory safety tools such as AddressSanitizer (ASan) to detect out-of-bounds writes during testing
• Monitor for anomalous crashes in applications known to use locale-specific number formatting
• Deploy intrusion detection signatures targeting exploitation patterns for glibc buffer overflows
• Use static analysis tools to identify sprintf usage with locale-dependent number formatting

Monitoring Recommendations

• Monitor system logs for repeated crashes in applications using glibc 2.37
• Implement canary values around buffers used for number formatting operations
• Set up alerts for memory corruption events detected by kernel-level protections (e.g., ASLR, stack canaries)
• Track glibc version inventory across systems to identify potentially vulnerable deployments

How to Mitigate CVE-2023-25139

Immediate Actions Required

• Inventory all systems running glibc 2.37 and prioritize patching based on exposure
• Upgrade to a patched version of glibc as soon as available from your distribution
• Consider adding additional buffer padding as a temporary defensive measure for critical applications
• Review applications that use sprintf with thousands separators and consider alternative formatting approaches

Patch Information

Organizations should consult their Linux distribution vendor for patched glibc packages. Additional information is available through:

• Sourceware Bug Report #30068 - Primary bug tracking and patch discussion
• NetApp Security Advisory NTAP-20230302-0010 - NetApp-specific guidance
• Openwall OSS Security Notice - Community security discussion

Workarounds

• Avoid using locale-specific thousands separators in sprintf calls until patched
• Add extra bytes (at least 2-4 bytes) to buffer allocations when formatting numbers with grouping characters
• Use snprintf with explicit bounds checking and monitor for truncation
• Consider using alternative formatting libraries that are not affected by this vulnerability

# Check glibc version on Linux systems
ldd --version

# Example: Add extra buffer padding as temporary mitigation
# Instead of exact calculation, add safety margin:
# buffer_size = calculated_size + 8;